You need some lessons in overclocking. First of all, disconnect your HDD when experimenting with system speeds or you get what you deserve when your HDD craps out. Second, you need to do some speed testing to see what each individual component in your system can handle.
http://12.18.239.202/heatercores/primer.htm
Read that and follow the testing procedure. Before you do that, though, put a better northbridge cooler on your board. Really, you can NOT expect a good measure of stability unless you swap that aluminum monstrosity off of your board.